Hitherto, image forming apparatuses have formed an image on a sheet conveyed in the apparatus. When a sheet is conveyed incorrectly, for example, a jam (paper jam) may occur and image formation may be not performed properly.
In some cases, to correct skew of a sheet which is being conveyed, the sheet is pressed against a registration roller pair so that the sheet bends.
When the sheet is bent, the leading edge of the sheet may be caused to fit into the nip of a registration roller pair by the elasticity of the sheet itself, thereby correcting the skew. After correction of the skew, conveying the sheet is restarted. A conventional image forming apparatus for adjusting the bending amount of a sheet in a conveying path will be described below.
In an image forming apparatus including an ultrasonic sensor, having a transmitting unit and a receiving unit being disposed opposite each other having an interval therebetween such that a sheet conveying path through which a sheet is conveyed is disposed therebetween, a first rotary member pair, and a second rotary member pair positioned downstream of the first rotary member pair in a conveying path, an upper edge of the sheet held and conveyed by the first rotary member pair is contacts the second rotary member pair and the sheet is stopped. The apparatus further includes the sheet conveying device for adjusting the stop timing of the first rotary member pair so that a value detected by the ultrasonic sensor does not exceed a predetermined threshold value.
In the above-described image forming apparatus, in some cases, the sheet is bent by a registration roller pair.
However, when the sheet is bent too much, folding of the sheet may occur. Whereas, if amount of bending is small, the skew of the sheet may not be corrected properly.
It is therefore necessary to control the bending amount of the sheet in order to properly convey the sheet.
For example, a fixing unit includes rotary member pair that sandwiches a sheet between the members. The fixing unit conveys the sheet while heating the sheet and applying pressure to the sheet in order to fix a toner image onto the sheet.
If a conveying speed of the fixing unit is slower than a speed at which a sheet is conveyed to the fixing unit, bending of the sheet may occur, thus causing the sheet to be folded in a Z-shaped pattern (hereinafter “Z-shaped folding”). The Z-shaped folding may also causes a jam in the fixing unit or in a position downstream of the fixing unit.
Typically, therefore, a sheet is bent in a position where bending of the sheet hardly causes an adverse effect on the conveyance of the sheet, e.g., a position upstream of the registration roller pair where conveying of the sheet is substantially insusceptible to the bending.
Controlling the bending amount of a sheet in the conveying path, as described above, is required for proper conveying of the sheet. Factors similarly causing a jam of a sheet include feeding of multiple sheets.
Multiple sheets feeding is a phenomenon caused by moisture or static electricity. Two or more sheets are fed from a paper feeding cassette or the like at a time adhering each other. If multiple sheets feeding occurs, the sheets are caught in a conveying path or various rotary members arranged in the conveying path, thus causing a jam.
Proper conveying of a sheet therefore requires detecting not only the amount of bending of a sheet but also the occurrence of multiple sheets feeding.
As for the related-art image forming apparatus, the amount of bending of a sheet can be detected by the ultrasonic sensor and a sensor flag. Unfortunately, such an apparatus needs the sensor flag and does not detect the amount of bending of a sheet accurately because the bending amount is smaller than 10 mm and the sonic wave runs about 340 meter per second. In order for such an apparatus to detect the bending amount of the sheet accurately, the apparatus needs a very accurate time measurement system which can measure time to an accuracy of a nanosecond, and thus, the apparatus becomes too expensive.